The invention relates to a process for separation of gases, and more specifically relates to separating one or more components from a multi-component gas stream using a semi-permeable membrane.
Many gas steams contain large amounts of acid gases, such as carbon dioxide, tat must be separated from the wore valuable components in the gas. One such gas stream is natural gas from well production, which is used extensively as fuel and as a basic raw material in the petrochemical and other chemical process industries. While the composition of natural gas can vary widely from field to field, many natural gas reservoirs contain relatively low percentages of hydrocarbons (less than 40%, for example) and high percentages of acid gases, principally carbon dioxides, but also hydrogen sulfide, carbonyl sulfide, carbon disulfide, and various mercaptans. Removal of the acid gases from well production is desirable to provide conditioned or sweet dry natural gas either for delivery to a pipeline, natural gas liquids recovery, helium recovery, conversion to liquid natural gas, or nitrogen rejection. The separated acid gases are available for processing, sequestration, or disposal. The acid gases have for example been reinjected into a subterranean formation for disposal and into s-hydrocarbon-bearing formations for hydrocarbon recovery.
A number of processes for the recovery or removal of carbon dioxide from gas streams have been proposed and practiced on a commercial scale. In practice, these processes occur at feed pressures below 1,200 psia (82.8 bar). The processes vary widely, but generally involve some form of solvent absorption, adsorption on a porous adsorbent, distillation, or diffusion through a semipermeable membrane.
Membranes are thin barriers that allow preferential passage of certain components of a multi-component gas mixture. Most membranes can be separated into two types: porous and nonporous. Porous membranes separate gases based on molecular size and/or differential adsorption by small pores in the membrane. Gas separation membranes used in natural gas applications are often nonporous or asymmetric and separate gases based on solubility and diffusivity. These membranes typically have a microporous layer, one side of which is covered with a thin, nonporous xe2x80x9cskinxe2x80x9d or surface layer. The separation of the gas mixtures through an asymmetric membrane occurs in its skin, while the microporous substrate gives the membrane mechanical strength.
In a typical membrane separation process, a gas is introduced into the feed side of a module tat is separated into two compartments by the permeable membrane. The gas stream flows along the surface of the membrane and the more permeable components of the gas pass through the membrane barrier at a higher rate than those components of lower permeability. After contacting the membrane, the depleted feed gas residue stream, the retentate, is removed from contact with the membrane by a suitable outlet on the feed compartment side of the module. The gas on the other side of the membrane, the permeate, is removed from contact with the membrane through a separate outlet. The permeate stream from the membrane may be referred to as being xe2x80x9cenrichedxe2x80x9d in the readily permeable components relative to the concentration of the readily permeable components late retentate stream. The retentate may also be referred to as being xe2x80x9cdepletedxe2x80x9d of the readily permeable components. While the permeate stream can represent the desired product, in most natural gas permeation processes the desired product is the retentate stream, and the permeate stream comprises contaminants such as CO2 or other acid gases.
While the selection of a suitable membrane typically involves many factors, two important factors are (1) the capability of the membrane to withstand the conditions to which it may be subjected during the separation operation, and (2) adequate selective separation of one or more desired gases at a sufficiently high flux (flow rate). Separation membranes that exhibit a high selectivity but low flux are unattractive as they require large separating membrane surface area. Similarly, separation membranes that exhibit adequately high flux but undesirable low selectivity are also unsuitable for practical application. Practical separation membranes are those having the capability of maintaining a desired performance of flux and selectivity over an extended period of time in adverse operating environments.
Membrane systems for removing CO2 from natural gas streams are typically designed to receive the natural gas stream at elevated pressure to avoid the costs associated with compressing the gas stream to a higher pressure level. If the produced gas pressure is above about 1,200 psia (82.8 bar), the conventional practice is to reduce the feed pressure to avoid damaging the membrane. An illustrative example of a membrane separation process is disclosed in U.S. Pat. No. 5,411,721 (Doshi et al.), which uses a membrane system to provide a high-pressure retentate stream rich in methane and a lower pressure permeate stream rich in CO2. Doshi et al. takes advantage of high wellhead gas pressure as the driving force for membrane separation by passing feed gas to a membrane system at pressures xe2x80x9cfrom 500 psia to about 2,000 psia or higherxe2x80x9d (34 to 138 bar). In contrast to the invention described in this patent, Doshi et al. is not concerned with performing high pressure membrane separation at elevated temperatures and it is not concerned with providing a high-pressure permeate.
An important aspect of any natural gas treating process is economics. Natural gas is typically treated in high volumes, making even slight differences in process efficiency very significant in the selection of process technology. The ability to perform acid gas separation at high pressure can increase that efficiency and have a large impact on the overall economics of the treating process. Some natural gas resources, particularly those with significant concentrations of non-hydrocarbons, are now uneconomical to produce because of processing costs. There is a continuing need for improved natural gas treating processes that have high reliability and represent simplicity of operation. It is particularly desirable to have a process that can effectively separate CO2 and other acid gases from natural gas so that the acid gas stream is at as high a pressure as possible for subsequent disposal or sequestration.
The invention relates to a method of separating one or more components from a multi-component gas stream comprising at least one non-acid gas component and at least one acid gas component. A multi-component gas stream at a pressure above 1,200 psia (82.8 bar) and a temperature above 120xc2x0 F. (48.9xc2x0 C.) with the concentration of at least one acid gas component in the gas stream being at least 20 mole percent is passed to a membrane system that selectively separates at least one acid gas component from the multi-component gas stream as a permeate stream. The permeate stream has a pressure of at least 20% of the pressure of the feed pressure.